In order to determine the mechanisms underlying the decrease in ventilation after the cessation of exercise, total circulatory occlusion will be produced at that time. If the time course of the decrease in or delivery of metabolites to central chemoreceptors, circulatory occlusion should change the pattern. If the decrement in ventilation is caused by a slowing of reverberatory activity in the respiratory centers, the pattern should be unchanged. Changes in the ventilatory transients to exercise will also be produced by changing inspired gases (hyperoxia and hypoxia) in order to evaluate the role of peripheral arterial chemoreceptors in the exercise response. BIBLIOGRAPHIC REFERENCES: "Human cardiorespiratory responses to impulses of work during circulatory occlusion". J. R. Hildebrandt and R.K. Winn. Physiologist 19:225, 1976.